1. Field of the Invention
The present invention relates to a printing apparatus and more particularly to a printing apparatus having a construction that makes efficient use of power capacity of the apparatus, and a print control method and a program for the printing apparatus.
2. Description of the Related Art
As information processing equipment, such as copying machines, word processors and computers, and communication devices proliferate, more and more digital image forming (printing) apparatus that employ an ink jet printing system in outputting information processed by these devices, such as characters and images, are coming into widespread use. Ink jet printing apparatus generally use a print head which has a plurality of ink ejection printing elements and ink paths integrated therein as printing elements for faster printing speed.
Of the ink jet printing apparatus, serial printers that are inexpensive and easily reduced in size have found growing market demands as personal printers. In such serial printers, the number of printing element operations greatly varies depending on the content of image data supplied from a host computer. For example, even in one and the same page of a print medium, there is a distribution in print density according to a print location. In this case, an electric power required for driving the print head also varies according to the print density. To allow for a stable printing without being affected by such power variations, it is necessary to provide a power supply with a relatively large capacity and a circuit that can withstand input and output to and from the large-capacity power supply. It is, however, feared that such a printer may become large in size and expensive.
In normal printing, an image that the print head prints at a maximum print density is only a very small portion of the entire page. In most of the page, lower density images are printed at lower drive frequency output. Thus, printers intended for low cost and small size adopt a method that controls a printing method according to the number of printed dots in a predetermined area to limit the amount of electricity consumed per unit of time. That is, in printing general image data a fast print mode is used and, in areas that require a high resolution printing, another printing method is used to ensure a high print quality.
For example, Japanese Laid-Open Patent Application No. 55-119784 (1980) discloses a printing method that seeks to save electricity in a printing apparatus which has a plurality of printing elements (nozzles) arrayed in a paper feed direction and scans a printing element column in a direction perpendicular to the paper feed direction to form an image. The printing apparatus disclosed here includes: a drive signal generation means for generating printing element drive signals for activating the printing elements; a counter means for using the printing element drive signals from the drive signal generation means as a count input to count the number of dots required to form a character in each block resulting from a division of one line; a detection means for determining which of preset multiple stages a count value of each block produced by the counter means falls in; and a selection means for selecting, based on an output of the detection means, a print speed for one line under consideration according to the stage of the count value produced by the counter means for a block in the line having the largest number of dots to be printed.
The technique disclosed in the above document has been proposed mainly for a wire dot system and a line printer and reduces the power consumption per unit of time by reducing the print speed for one line.
Some other printers employ a method which divides data for an area that can be printed at one printing scan into two or more blocks of data and scans the print head a plurality of times in that area. For example, Japanese Patent Application Publication No. 6-47290 (1994) (U.S. Pat. No. 4,653,940) discloses a technique that prints one line in a plurality of scans when that line has an area with a dot duty in excess of a predetermined dot duty. As one method of printing one line in a plurality of scans, this document proposes printing the line in two scans by dividing the printing elements into even-numbered printing elements and odd-numbered printing elements.
There is another method for dividing an area that can be printed by the print head at one printing scan into two blocks of area. For example, all the printing elements in the print head may be divided into an upper half block and a lower half block, and the printing scan may also be divided into a first printing scan using only the upper half block and a second printing scan using only the lower half block. Another method involves using two print masks that are complementary to each other (here, they are grid-shaped) and completing an image area D in two printing scans. The difference among these methods lies in the way the printing of one and the same image area is divided into two printing actions.
This method does not limit to two the number of times that the printing operation is divided. The above document has a description to the effect that the number of divided printing operations is not limited to two but the printing operations may be divided into two or more.
Further, Japanese Patent No. 3376118 (U.S. Pat. No. 6,155,663) discloses a method of dividing data and performing a plurality of printing scans to complete an image in one area.
However, the conventional method described above, which, as in the divided printing, simply switches to a power save mode when the predetermined number of printed dots is exceeded, can hardly be said to utilize the power supply efficiently at all times within its service range. That is, since the print mode switches to the power save mode if the number of dots exceeds a threshold even by a small margin, the power supply is hardly used to near its maximum usable capacity and there is a possibility of only the printing time (throughput) increasing significantly.
In the method that reduces the speed of the printing scan to prevent the power consumption from exceeding the maximum capacity of the power supply, it is very difficult from the aspect of a motor performance to stably operate a print head scanning drive motor in a wide range from high to low speed. That is, the printing speed cannot be lowered unlimitedly according to the current flow and it is therefore required to select a scan speed within a range in which the drive motor can operate stably.